1. Field of the Invention
The invention relates to a varactor and, more particularly, to a varactor device capable of successively tuning its capacitance magnitude in a large range.
2. Description of Related Art
Generally, a varactor is completed by a MOS process. As shown in FIG. 1, the MOS varactor has a capacitance tuning ratio which is lower than three and needs complicated masks in production, thus it cannot meet the practical requirement. FIG. 2 is a view of a typical diode varactor produced by a CMOS process, which has a capacitance tuning which is ratio lower than two, thus it can tune the capacitance only in a small range and cannot meet the practical requirement as well.
In addition, a digital varactor is designed, as shown in FIG. 3. The digital varactor consists of capacitors CF, CD0˜N, and switch transistors S0˜N The switch transistors S0 ˜N control the respective capacitors CD0˜N to be connected in parallel with the capacitor CF or not. Accordingly, a number of capacitors connected in parallel are changed by respectively controlling the switch transistors S0˜N on and off. Thus, the capacitance tuning effect is obtained. In addition, the capacitance tuning ratio can relatively increase with the increased number of the capacitors CD0˜N However, such a tuning can tune the capacitance only to a specific value rather than tuning the capacitance successively.
Therefore, it is desirable to provide an improved varactor device to mitigate and/or obviate the aforementioned problems.